U.S. patent number 4,703,650 [Application Number 06/877,844] was granted by the patent office on 1987-11-03 for circuit for the coding of the value of two variables measured in a tire, and device for monitoring tires employing such a circuit.
This patent grant is currently assigned to Compagnie Generale des Etablissements Michelin. Invention is credited to Andre Dosjoub, David Myatt.
United States Patent |
4,703,650 |
Dosjoub , et al. |
November 3, 1987 |
Circuit for the coding of the value of two variables measured in a
tire, and device for monitoring tires employing such a circuit
Abstract
A coding circuit comprises an astable multivibrator which
transforms the measurement of the variables in question, for
instance pressure and temperature, into a time measurement. The
astable multivibrator delivers a pulse signal whose pulse width is
a function of the temperature and the cyclic ratio of which is a
function of the pressure.
Inventors: |
Dosjoub; Andre (Chamalieres,
FR), Myatt; David (Pompignat, FR) |
Assignee: |
Compagnie Generale des
Etablissements Michelin (Clermont-Ferrand, FR)
|
Family
ID: |
9321127 |
Appl.
No.: |
06/877,844 |
Filed: |
June 24, 1986 |
Foreign Application Priority Data
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Jul 3, 1985 [FR] |
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85 10515 |
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Current U.S.
Class: |
73/146.5; 73/726;
73/714; 374/143 |
Current CPC
Class: |
B60C
23/0427 (20130101) |
Current International
Class: |
B60C
23/04 (20060101); B60C 23/02 (20060101); B60C
023/04 (); G01L 009/04 () |
Field of
Search: |
;73/146.5,152,766,726,727,753,714 ;374/143 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0016991 |
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Mar 1980 |
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EP |
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2122757 |
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Jul 1983 |
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GB |
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Primary Examiner: Woodiel; Donald O.
Attorney, Agent or Firm: Brumbaugh, Graves, Donohue &
Raymond
Claims
What is claimed is:
1. A circuit for coding the value of two variables measured in a
tire, which permits the transmission of the values to the chassis
bearing the tire without galvanic contact, characterized by the
fact that the circuit comprises:
(a) a first unit the transfer function of which is a function of
the first of the variables measured;
(b) means for alternately connecting to the input of the first unit
either a reference voltage or a voltage which is a function of the
second of the variables measured to provide an output signal
representing either said first variable or said second variable,
respectively; and
(c) a second unit controlled by the output signal of the first unit
and delivering a pulse signal having parameters representing the
values of the measured variables,
said connecting means being actuated in response to said pulse
signal.
2. A circuit according to claim 1, characterized by the fact that
the first unit comprises essentially an integrator and by the fact
that the time constant of the first unit is a function of the first
measured variable.
3. A circuit according to claim 1 or 2, characterized by the fact
that the second unit comprises essentially means permitting the
referencing of two thresholds for the output signal of the first
unit.
4. A circuit according to claim 3, characterized by the fact that
the means permitting the referencing comprise two comparators and a
flipflop.
5. A circuit according to claim 3, characterized by the fact that
the means permitting the referencing comprise a single comparator
provided with a feedback loop.
6. A circuit according to claim 1 or 2, characterized by the fact
that the voltage which is a function of the second of the variables
measured is applied to the input of the first unit via an
operational amplifier receiving a voltage delivered by a detector
responsive to the second variable.
7. A circuit according to claim 6, characterized by the fact that
said second variable is the pressure prevailing in the tire and by
the fact that the detector is a piezoresistive gauge.
8. A circuit according to claim 1 or 2, characterized by the fact
that the connecting means are formed of a transistor mounted as a
common emitter the base of which is controlled by the output of the
second unit and the collector of which is joined to the input of
the first unit.
9. A circuit according to claim 1 or 2, characterized by the fact
that it is essentially in the form of a hybrid circuit or
integrated circuit.
10. A circuit according to claim 2, characterized by the fact that
the integrator is formed of a connection which comprises at least
one operational amplifier, a resistor being connected to the
inverting input and a condenser being inserted in the feedback
circuit, a voltage being applied to the non-inverting input of the
operational amplifier by a resistance divider bridge permitting
calibration of the the circuit relative to the second measured
variable.
11. A circuit according to claim 10, characterized by the fact that
the first variable is the temperature prevailing in the tire and by
the fact that a temperature detector is used in the form of a
silicon resistor mounted in the feedback loop of an additional
operational amplifier, the output of which acts on said one
operational amplifier in the integrator.
12. A circuit according to claim 10, characterized by the fact that
the first variable is the temperature prevailing in the tire and by
the fact that a temperature detector is used in the form of a
silicon resistor mounted at the output of said one operational
amplifier in the integrator.
Description
BACKGROUND OF THE INVENTION
The present invention relates to devices for the monitoring of
tires. More particularly, it relates to the transmission of the
pressure and temperature of the tire to the chassis of the vehicle
without galvanic contact.
The state of the art contains numerous attempts directed at
advising the driver of a vehicle of a decrease in the pressure of
one of his tires. U.S. Pat. No. 4,389,884 proposes that the
detection be effected by means of bellows which move a ferrite
within a coil in order to modify the value of the inductance
thereof as a function of the pressure. The transmission is effected
by inductive coupling between two coils, one fastened to the rim
and the other to a part of the vehicle which is not movable in
rotation. European Pat. No. 45,401 contemplates feeding an active
circuit arranged in the tire with energy, modulating an electric
signal in frequency as a function of the pressure, transmitting the
signal towards the chassis by inductive coupling, and analyzing
this signal as a function of various parameters. Among the various
arrangements which independently monitor two or more variables
within the tire, mention may be made of U.S. Pat. No. 4,052,696
which describes an arrangement which gives a warning when a
pressure threshold or temperature threshold is exceeded.
The known devices have proven unable to assure reliable and precise
monitoring of the condition of a tire. The behavior of a tire is,
as a matter of fact, very complicated and the monitoring thereof
cannot be reduced to the simple detection of a threshold. It is
desirable to know continuously the pressure and the temperature
prevailing within the tire. As already stated, it is not sufficient
to transmit a threshold-exceeding signal of the parameters
considered; it is necessary for a monitoring device to be able to
transmit a measurement of said parameters, the measurements
transmitted being capable of use for direct reading or being
processed by an analysis system which controls a warning unit
located on the instrument panel.
SUMMARY OF THE INVENTION
The object of the invention is to permit the construction of an
arrangement for monitoring the condition of a tire which is capable
of transmitting the value of two variables measured within the
tire.
Another object of the invention is to provide a circuit for coding
the value of two variables measured in a tire which permits
transmission of the values to the chassis of the vehicle without
galvanic contact.
A further object of the invention is to provide a monitoring
device, equipped with such a circuit, which takes up as little
space as possible, which is without mechanical parts and which is
simple and reliable.
According to the invention, the circuit for coding the value of two
variables measured in a tire, which permits the transmission of the
values to the chassis bearing the tire without galvanic contact, is
characterized by the fact that the circuit comprises:
(a) a first unit the transfer function of which is a function of
the first of the variables measured;
(b) means which make it possible to connect to the input of the
first unit either a reference voltage or a voltage which is a
function of the second of the variables measured;
(c) a second unit controlled by the output signal of the first unit
and delivering a pulse signal the parameters of which bear the
values of the measured variables.
By "pulse signal" there is understood a rectangular wave whose
"parameters" are the width of the period during which this signal
is in high state and the width of the period during which the
signal is in low state, or else one of these two widths and the
cyclic ratio of the signal.
DESCRIPTION OF THE DRAWINGS
The accompanying drawings will make it possible to understand more
readily the invention and to grasp all of its advantages. They show
two non-limitative embodiments of the invention applied to the
measuring of the pressure and temperature prevailing within a
tire.
FIG. 1 is a synoptic diagram of the coding circuit of the
invention.
FIG. 2 is a diagram of the circuit in accordance with the first
embodiment.
FIGS. 2a to 2f are time diagrams of the principal signals, serving
to explain the operation of the coding circuit shown in FIG. 2.
FIG. 3 is a diagram of the circuit in accordance with another
embodiment.
FIGS. 3a to 3f are time diagrams of the principal signals of the
coding circuit shown in FIG. 3.
DETAILED DESCRIPTION OF THE INVENTION
The principle of operation of the coding circuit shown in FIG. 1 is
to transform the measurement of the variables in question into time
measurements by means of an astable multivibrator 1 which delivers
a pulse signal V.sub.o whose pulse width is a function of the first
of the variables measured, namely the temperature of the tire in
the examples considered, and the cyclic ratio of which is a
function of the second of the variables measured, namely the
pressure of the tire in these examples.
The multivibrator 1 comprises a first unit 2 whose transfer
function is that of an integrator, receiving at its input either a
reference or a signal which is an image of the second variable
measured, and then a second unit 4 which transforms the output of
the first unit 2 into a pulse signal. This coding circuit therefore
permits the simultaneous transmission of two values, one coded by
the cyclic ratio of the pulse signal and the other by the width of
a pulse. However, this application is not limitative; with the same
circuit one can just as well transmit successively in time first of
all the values of two measured variables and then the values of two
other measured variables.
Referring now to FIG. 2, it is seen that the coding circuit
comprises a first unit 2 formed essentially of an integrator 20,
the time constant of the unit 2 being a function of the first of
the variables measured (in this example, the first variable is the
temperature). This integrator 20 comprises a circuit containing at
least one operational amplifier AO3, a resistor R.sub.o being
connected to the inverting input and a condenser C.sub.o being
inserted in the feedback circuit. This is the classic manner of
using an operational amplifier as integrator. A voltage is applied
to the non-inverting input of the operational amplifier AO3 via a
resistance divider bridge formed of the resistors R.sub.1 and
R.sub.2, the calibrating role of which as a function of the second
variable measured is explained further below. A silicon temperature
detector R.sub..theta. is used the characteristics of which
(internal resistance, sensitivity, linearity, reliability)
correspond precisely to the needs of this application. This
detector R.sub..theta. is mounted in the feedback loop of an
additional operational amplifier AO2 connected as amplifier the
output of which acts on the operational amplifier AO3 connected as
integrator via the resistor R.sub.o. The voltage delivered by the
resistance divider bridge formed of the resistors R.sub.1 and
R.sub.2 is applied also to the non-inverting input of the
operational amplifier AO2.
The integrator (i.e. the first unit 2) delivers a voltage V.sub.I.
This voltage V.sub.I is the input signal of a unit 4 which permits
the referencing of two thresholds, in this case by means of two
comparators 41 and 42, a resistance divider bridge formed of three
resistors r connected in series to deliver the threshold voltages,
and a flipflop 43. By "referencing of two thresholds" there is
understood the locating of the times when the signal V.sub.I in
question reaches one of said thresholds; this unit delivers a pulse
signal which bears values of the variables measured.
The coding circuit furthermore comprises a transistor 3 connected
with common emitter, the base of which is controlled by the output
signal of the flipflop 43 and the collector of which is connected
to the input of the first unit 2. An operational amplifier AO1
delivers a signal V.sub.S which is also applied to the input of the
first unit 2 via resistor R the role of which is to avoid
short-circuiting the output of the operational amplifier AO1 when
the transistor 3 is open. The operational amplifier AO1 is
connected as a differential amplifier. At the input it receives a
voltage delivered by a detector which is sensitive to the second
measured variable, namely the pressure in this embodiment of the
coding circuit. It is important that the detector selected be
electrically equivalent to a Wheatsone bridge or, more generally,
deliver a voltage which is proportional to the variable measured.
As pressure detector 5 there is preferably used a piezoresistive
gauge the gauge factor of which is higher than that of a
conventional gauge of the metallic type.
The operation of the coding circuit will now be explained with more
particular reference to FIGS. 2a to 2f which show time diagrams of
the principal signals present in the circuit.
By virtue of the pressure-detector selected, which is sensitive to
the absolute pressure, the voltage v delivered varies
proportionally between zero and a value corresponding to the
maximum pressure for which the monitoring device is designed, which
is indicated in FIG. 2a by the dashed line marked with the
parameter "p.sub.max ". Let us consider the value of the signal v
indicated by the solid horizontal line marked "p.sub.measured " (we
will assume that the pressure remains constant during the period of
time considered). FIG. 2b shows the signal V.sub.S, which varies
between the levels of the parameters "p.sub.max " and "p=0." As a
function of the dimensioning of the range of operation of the
operational amplifier AO1, let us consider that the limit values of
V.sub.S are
(V.sub.A being the feed voltage of the coding circuit), which in no
way reduces the generality of the demonstration. The following
relationship is immediately established:
FIG. 2c gives a time diagram of the signal I.sub.1, the current
passing through the resistor R which is arranged at the input of
the first unit 2. During the period of time T.sub.p, the transistor
3 is blocked and this current is a function of the difference in
potential between the voltage at the inverting input of the
operational amplifier AO2 and V.sub.S. During the period of time
T.sub.o, the transistor 3 is unblocked and therefore the current
I.sub.1 is a function of the voltage at the inverting input of the
operational amplifier AO2 since the reference here is the ground.
By application of the rules of operation of operational amplifiers,
the voltage at the input of the operational amplifier AO2 (and
furthermore of the operational amplifier AO3) is adjusted so that
the current I.sub.1 is zero if the absolute pressure is zero. As a
variant, the current I.sub.1 corresponding to an absolute pressure
of zero can be adjusted to any value. By reference to FIG. 2b and
to the above explanations it is seen that said voltage is V.sub.A
/3). Thus the coding circuit is calibrated relative to the
pressure. Using the expression for V.sub.S, it is immediately
deduced that:
during the period of time T.sub.p, ##EQU1## during the period of
time T.sub.o,
FIG. 2d shows the current I.sub.2 passing through the resistor
R.sub.o. By a simple application of the fundamental rules of
operation of the operational amplifier AO2 connected as amplifier
it is calculated, respecting the polarities selected, that
which gives
during the period of time T.sub.p ##EQU2## during the period of
time T.sub.o ##EQU3##
FIG. 2e shows the law of variation of the voltage V.sub.I at the
output of the integrator 20. As the operational amplifier AO3
operates as an integrator, the voltage at the terminals of the
condenser C.sub.o is in accordance with the law ##EQU4## Thus,
during the period of time T.sub.p, the variation of V.sub.I as a
function of time is represented by an ascending ramp, while during
the period of time T.sub.o it is a descending ramp. We may note
that the slope of the ascending ramp depends both on the pressure
(p in Equation 1) and on the temperature (R.sub..theta. in Equation
1) while the slope of the descending ramp depends only on the
temperature (R.sub..theta. in Equation 2).
The limit values of V.sub.I are established by the thresholds
referenced by the comparators 41 and 42 ##EQU5## respectively).
Let us consider the signal V.sub.I during the period T.sub.p. As
soon as V.sub.I exceeds the value (2.sup.V A/3), the comparator 41
sends a pulse to the S (set) input of the flipflop 43 which
therefore delivers a high signal which, applied to the transistor
3, unblocks it. The input of the first unit 2 is therefore
grounded, which has the effect of causing the manner of operation
described for the period T.sub.o to start. As soon as the signal
V.sub.I becomes less than (V.sub.A /3) the comparator 42 sends a
pulse to the R (reset) input of the flipflop 43 which therefore
delivers a low signal (zero volt), which blocks the transistor 3.
The operating mode described for the period T.sub.p starts. The
form of the signal V.sub.I makes it possible to write ##EQU6##
The above equations show that the periods T.sub.o and T.sub.p are
independent of the feed voltage V.sub.A, which is still subject to
fluctuations for a circuit intended to be implanted on a wheel,
whatever the manner of feed, by storage cell (progressive
discharge) or by condenser charged by inductive coupling, as
indicated in European Pat. No. 45,401 (rapid discharge). It follows
from Equation 4, using the resistance-temperature characteristic of
the detector R.sub..theta., that the temperature measured is a
function of the period of time T.sub.o. By introducing into
Equation 3 the value R.sub..theta. obtained from Equation 4, one
obtains, by a few elementary calculations, the expression for the
pressure ##EQU7##
It is therefore seen that the measurement of two variables has been
replaced by time measurements, which are furthermore independent of
the feed voltage of the circuit which lends itself particularly
well to the reliable transmission of the measured variables between
a wheel and the chassis of a vehicle.
FIG. 2f shows the output signal V.sub.o of the coding circuit of
FIG. 2. The output signal is actually that of an astable
multivibrator since, in practice, one does not measure pressures
less than atmospheric pressure and therefore the current I.sub.2 is
never zero. Thus the temperature is a function of the pulse width
of the signal V.sub.o and the pressure is a function of the cyclic
ratio of said signal V.sub.o. Electronic components of CMOS
technology are preferably employed in order to minimize the circuit
consumption. The circuit can be formed with discrete components or,
preferably, in the form of a hybrid or integrated circuit for most
of these elements.
Another embodiment of the coding circuit according to the invention
is proposed in FIG. 3. The first unit 2, whose transfer function is
a function of the first of the measured variables, comprises only a
single operational amplifier connected as integrator A2, and then,
at the output of this amplifier, a temperature-sensitive resistor
R.sub..theta.. There is then a second unit which makes it possible
to effect the referencing of two thresholds by means of a single
comparator 40. For this purpose, the comparator receives, on the
one hand, a reference voltage formed of a fraction of the feed
voltage namely k.sub.2 V.sub.A, and, on the other hand, a signal
which is a function both of the output of the integrator A.sub.2,
and therefore of the output of the first unit 2 and of the output
of the second unit 4, in its turn via the feedback loop formed by
the branch containing the resistor R.sub.8.
This second example is based on the same principle as the first and
it is therefore needless to repeat the entire operation in detail.
The following explanation should be consulted in parallel with
FIGS. 3a to 3f which relate to the principal signals of the
circuit.
The signal V.sub.S is processed in the same manner as in the first
variant.
The branches containing the resistors R.sub.5 to R.sub.7 constitute
a resistive voltage divider of the feed voltage V.sub.A. Taking
into account the sign convention selected for the current I.sub.1
in FIG. 3 and considering the voltage V.sub.j at the output of the
integrator A2, one immediately has ##EQU8## during the period of
time T.sub.p (transistor 3 blocked): ##EQU9## during the period of
time T.sub.o (transistor 3 unblocked): ##EQU10##
FIG. 3c represents the law of variation of I.sub.1, taking into
account the fact that k.sub.1 must be so selected that k.sub.1
V.sub.A -V.sub.S is negative.
The second unit 4 comprises a comparator 40 whose output is in the
low state (namely V.sub.o =zero volt) when the voltage applied to
the non-inverting input (namely V.sub.+) is less than k.sub.2
V.sub.A (which is true for the period T.sub.o when the output of
the operational amplifier AO2 is increasing) and whose output is in
the high state (namely V.sub.o =V.sub.A) when V.sub.+ is greater
than k.sub.2 V.sub.A. Based on the sign conventional adopted for
the current I.sub.3 in FIG. 3, let us calculate the trigger points
of the comparator 40 (see A and B in FIG. 3d):
starting from V.sub.o =0, one has at B ##EQU11## starting from
V.sub.o =V.sub.A, one has at A: ##EQU12##
It is therefore seen that the signal V.sub.o as a function of
I.sub.3 comprises hysteresis, introduced by the reaction loop
containing the resistor R.sub.8. This permits the referencing of
two thresholds with a single comparator 40.
Let us calculate the variation .DELTA.I.sub.3 between the two
thresholds A and B: ##EQU13## By a simple application of Ohm's law
and by successive substitutions we have: ##EQU14##
Equations 5 and 6 show that the periods T.sub.o and T.sub.p are
this time still independent of the feed voltage. From Equation 6 it
follows that the temperature (knowing the resistance-temperature
characteristic of the detector R.sub..theta.) is measured by
T.sub.o. By introducing the value of R.sub..theta. derived from
Equation 6, one obtains, after a few elementary calculations, the
result that the pressure is measured by the cyclic ratio T.sub.o
/T.sub.p.
FIG. 3f shows the output signal of the coding circuit described in
FIG. 3. This signal is equivalent to that supplied by the first
embodiment (FIG. 2f) since, as far as logical levels are concerned,
it is immaterial whether V.sub.o is at the high state or at the low
state during the period T.sub.o. In FIG. 3 one again finds an
invertor 30 which makes it possible to control the transistor 3 in
suitable manner.
The means of feeding the coding circuit as well as the means of
transmitting the coded signal V.sub.o are known to the man skilled
in the art and do not fall within the scope of the present
invention. By way of example, in French Patent Application No.
85/10516 (corresponding to U.S. application Ser. No. 878,060 filed
concurrently herewith--Attorney's File) of the present Applicants
one finds the description of an energy feed stage and a signal
transmission stage by inductive coupling, making it possible
further to improve the overall performance of a tire monitoring
system. The type of signal transmitted requires that the
transmission not depend on the angular position of the wheels. In
the case of transmission by inductive coupling, it is therefore
necessary that the coil connected to the wheel be concentric to it.
However, the tolerance for the positioning of the coils (one
connected to the wheel and the other to a part of the vehicle which
is not movable in rotation) may be rather large. The frequencies
transmitted by inductive coupling should preferably be less than
100 kHz in order to avoid too extensive an attenuation.
* * * * *